Using NASA's Hubble Space Telescope, astronomers
watching the comet LINEAR (C/1999 S4) in July 2000
were surprised to catch the icy comet in a brief,
violent outburst when it blew off a piece of its crust,
like a cork popping off a champagne bottle.

The eruption, the comet's equivalent of a volcanic
explosion, spewed a great deal of dust into space
(even though temperatures are far below freezing,
at about minus 100 degrees Fahrenheit in the icy regions
of the nucleus of core). This mist of dust reflected sunlight,
dramatically increasing the comet's brightness over
several hours. Hubble's sharp vision recorded the entire
event and even snapped a picture of the chunk of
material jettisoned from the nucleus and floating away
along the comet's tail. The renegade fragment moved away
from the core's weak gravitational grasp at an average
speed of about 6 mph.

Scientists have identified several theories for the
eruption. One possible reason is that a particularly
volatile region of the core became exposed to sunlight
for the first time and vaporized suddenly. Another
possibility is that a buildup of gas pressure from
sublimating ice (a change from ice to gas) trapped just
below the comet's surface explosively blew the lid off
a pancake-shaped layer of crust from its surface. The
pressure from sunlight blew the fragment down the tail.
Yet another possibility is that the observed fragment is
one of the house-sized "cometesimals" that are thought
to make up the nucleus. Evidence accumulated during the
past decade suggests that comet nuclei are "rubble
piles" of loosely held together cometseimals. Perhaps
one of the "building blocks" comprising the core broke
off and was blown down the tail by a gaseous jet
shooting off the comet's surface like a garden hose
spray.

Though comet nuclei have been known to fragment,
the view from the Hubble telescope is revealing finer
details of how they break apart. This unexpected glimpse
at a transitory event may indicate that these types of
"Mt. Saint Helens" outbursts occur frequently on the
comet.

The orbiting observatory's Space Telescope Imaging
Spectograph tracked the streaking comet from
July 5 to 7, capturing the leap in brightness and
discovering the castaway chunk of material sailing
along LINEAR's tail. When the Hubble telescope first
spied the comet 74 million miles from Earth, the icy
object's brightness rose by about 50 percent in less
than four hours. By the next day, the comet was 1/3
less luminous than it was the previous day, and on the
final day, the comet was back to normal.

A week later, on July 14, NASA's Chandra X-ray
Observatory imaged the comet and detected X-rays from
oxygen and nitrogen ions. The details of the X-ray
emission show that the comet produces X-rays due to the
exchange of electrons in collisions between nitrogen
and oxygen ions in the solar wind and electrically
neutral elements (predominantly hydrogen) in the
comet's atmosphere. To further examine these findings,
the comet LINEAR will be re-observed with Chandra from
July 29 - August 13, 2000.

Hubble observations of comet LINEAR also measured a
deficiency of carbon monoxide compared with other
comets, which suggests that the comet origianlly formed
much closer to the Sun at temperatures that would have
depleted the carbon monoxide. The comet was then tossed
out to the Oort cloud, a vast and distant "deep freeze"
reservoir of primordial comet nuclei.

Comet LINEAR is named for the observatory that
originally discovered it in September 1999. LINEAR is
the acronym for Lincoln Near Earth Asteroid Research,
a project based in Lexington, Massachusetts.